The known types of systems for satellite navigation on the global scale, and more particularly the system known as GPS (for “Global Positioning System”), comprise, as shown in FIG. 1 placed at the end of the present description, a constellation of satellites 1, in orbit around the Earth, which are in communication with ground control stations 2, in order to make navigation signals available to user receivers 3.
The various satellites 1 are synchronized with each other by the control stations 2, which are themselves linked together. These stations calculate the orbital parameters of the satellites and transmit them to them, as shown symbolically by the arrow 4, by way of the stations 2.
The signals sent out by the satellites 1 comprise, in essence, positioning and time data. As illustrated diagrammatically by the arrow 5, a user receiver 3 picking up these positioning signals as sent out by at least four separate satellites deduces therefrom the exact time and his exact position with respect to the satellites 1 at the instant of the measurement and, from, that, his absolute position with respect to any ground reference system.
In practice, at least for the American GPS system, the precision on the position information obtained can be limited at any moment to about a hundred meters by scrambling of the signals which is deliberately imposed by the American military authorities, and, even when the precision available is of the order of about ten meters in the case of civil users, it remains inferior to that available to the military users.
It has also been proposed to add to these systems geostationary satellites 6 capable, moreover, of directly relaying (arrows 7, 8) information delivered by the ground control stations 2 to the user receivers 3.
Finally, the system may also include “pseudolite” ground stations 9, that is to say ground stations the position of which is known accurately in a local environment and which, as regards a user arriving close by, behave in the scheme of things like a satellite. These stations, via differential measurements, make it possible locally to increase the precision of the position down to less than ten meters, this for non-military users. These may include, for example, a station in proximity to a runway receiving the navigation signals (or positioning signals) sent out by the satellites in order to determine any positioning error which the user receiver will have to correct and to relay the information to the latter.
One major defect of satellite navigation systems is that the users cannot count on any guarantee as to the quality of the service offered, neither as regards the precision of the information received nor as regards the very availability of the signals. This is because the receivers necessary for interpreting the signals sent out by the satellites are freely available on the market, such that, in normal times, the system can be used by anyone who possesses such a receiver, without it being possible to verify what use is made of it. It results therefrom that it always has to be feared that a manager of the system might modify its operation, or even go as far as cutting it off completely, as the Armed Forces could do, for example, in times of crisis, without restricting themselves to reserving increased precision by comparison with civil users.
Assured availability of the signal with a known precision better than 10 meters would, however, be currently necessary, outside the military field, for many professional activities, such as the landing of aircraft in poor visibility conditions, or maritime navigation in ports or shallow waters. The known satellite navigation systems do not make it possible to satisfy these users. In consequence, the use of a system such as the existing GPS system, for example, is not currently allowable as a single means of navigation for operations having a safety-critical character. A limited use is permitted only with the support of conventional navigation facilities.
From the absence of an assurance of availability of the signals, as well as from the absence of a guaranteed precision (it is variable and unknown to the user), there also result drawbacks of an economic nature. This is because the GPS system has no provision for making it possible to have the use of the service paid for. This results, furthermore, from the fact that it is not possible to know who is using the system.
Finally, even for the military users, protection against unauthorized accesses depends largely on the degree of protection of the military receivers and of the access codes. It is not possible to know whether these codes have not been violated and whether unauthorized users are using the system with the aid of these violated codes.